Formation mechanisms of two-dimensional nanostructures in wet syntheses are poorlyunderstood. Even more enigmatic is the influence of hydrodynamic forces. Here we use liquidflow cell transmission electron microscopy to show that layered double hydroxide, as a modelmaterial, may form via the oriented attachment of hexagonal nanoparticles; under hydro-dynamic shear, oriented attachment is accelerated. To hydrodynamically manipulate thekinetics of particle growth and oriented attachment, we develop a microreactor with high andtunable shear rates, enabling control over particle size, crystallinity and aspect ratio. Thiswork offers new insights in the formation of two-dimensional materials, provides a scalableyet precise synthesis method, and proposes new avenues for the rational engineering andscalable production of highly anisotropic nanostructures.